1. Field of the Invention
This invention relates to spreading pastes containing polyurethane plastics and to their use as a new process for the production of coatings permeable to water vapor on fabric or leather substrates by the direct or transfer method based on the principle of evaporation coagulation.
2. Description of the Prior Art
Fabric or leather coatings which are highly permeable to water vapor and, at the same time, highly waterproof are of increasing interest, above all in the shoe and clothing fields. The advantages of coated materials such as these is that the clothing produced from them on the one hand affords optimal protection, but on the other hand allows bodily moisture to escape in the form of water vapor. These properties are extremely valuable both from the sanitation and hygiene point of view and also from the point of view of wearing comfort.
An important process for producing microporous coatings permeable to water vapor is the so-called bath coagulation process described, for example, in DE-AS No. 1,270,276 and in DE-AS No. 1,769,277. In this process, a substrate is coated with a polyurethane or polyurethane urea dissolved in an organic solvent and the coated product is introduced into a bath of a non-solvent (for example, water) miscible with the solvent. Coagulation of the polyurethane is obtained by extraction of the solvent by the non-solvent.
The disadvantages of this process are that very large quantities of non-solvent are required for the necessary complete removal of the solvent and that the process is time-consuming. In addition, special, relatively expensive apparatus is required for carrying out this process and also for working up the solvent/non-solvent mixtures accumulating therein.
Accordingly, there have been no shortage of attempts to produce coatings permeable to water vapor by simple direct or transfer coating using conventional fabric coating machines. Most of these attempts are based on the technique of evaporation coagulation. In principle, this process is carried out by adding to a polymer dissolved in a volatile solvent a certain quantity of a less volatile non-solvent and spreading the resulting solution, dispersion or suspension onto a substrate. The coating is dried by careful heating, during which the volatile solvent preferentially evaporates first. The result is that the polymer coagulates in the layer and, after final drying, shows a microporous structure. This process is described, for example, in DE-PS No. 1,694,059. The polyurethanes used therein are colloidally dissolved in volatile organic solvents, such as for example tetrahydrofuran or methylethylketone, and are mixed with organic non-solvents having a higher evaporation index, such as cleaning spirit. A similar process is described in CH-PS No. 481,971 with the added feature that water is included among the non-solvents mentioned therein for addition to the polymer solutions. Another example of this approach is the process according to DE-PS No. 2,004,276 which uses hydrophilic polyurethanes based on aromatic diisocyanates which contain certain proportions of polyoxyethylene compounds as synthesis components. Solutions of these polyurethanes in certain organic solvents, such as methylethylketone for example, are mixed with water as non-solvent and applied to a substrate, after which the coating is coagulated by selective evaporation and subsequently dried.
However, the above-mentioned processes based on the principle of evaporation coagulation also have serious disadvantages. A major disadvantage is that selective evaporation of the more volatile solvent components is time-consuming and requires extremely precise temperature control. Accordingly, handling of the corresponding products in the coating machines is complicated and, above all, only possible at slow speeds. Another serious disadvantage which applies, in particular to the process according to DE-PS No. 2,004,276, is that the polyurethane solutions or suspensions described therein are difficult to process. Although they have low solids contents, these products are highly viscous, even before the addition of water which is made at the time of application. They are described as "sludge-like suspensions", show a pronounced tendency towards premature drying with formation of gel particles and specks and are therefore difficult to handle by the coater. In addition, the coatings according to the last of the above-mentioned patents are attended by the disadvantage of poor fastness to light.
Accordingly, the object of the present invention is to provide a process for the production of coatings permeable to water vapor which may be carried out by the direct or transfer method in conventional coating machines and which does not have any of the disadvantages described in the foregoing.
This object is achieved by the multi-phase stable spreading pastes according to the invention which are described in detail in the following and by their use for coating by the direct or transfer method on the principle of evaporation coagulation.
The invention is based on the surprising observation that multi-phase stable spreading pastes which may be processed surprisingly easily, quickly and safely in conventional coating machines to form microporous coatings can be obtained from basically hydrophobic polyurethanes or polyurethane ureas in which certain silicone, polyether, polyester or perfluorocarbon resin segments are incorporated and which are dissolved in organic solvents, more especially by the addition of certain non-polyurethane resins containing lateral or terminal carboxylate groups which are defined in detail hereinafter. The base polymers according to the invention are preferably polyurethane ureas based on aliphatic polyisocyanates so that light-stable coatings permeable to water vapor are advantageously obtained.
The process according to the invention has the advantage that the polyurethane solutions are low in viscosity before addition of the water and, accordingly, are easy and safe to handle by the coater who only prepares the ready-to-use spreading pastes therefrom shortly before application. In addition, the spreading pastes produced by the process according to the invention by the addition of resin containing carboxylate groups and water are distinguished by particularly good stability in storage and are safe to process.